I rented and tried the 14mm f2.8 L II on my 6D. I think its one of the best lens from Canon in terms of optics.

But then its too damn expensive & I could not figure out if its worth the price. (2000+ USD)And it has a bulged front end but not a fish eye lens.

When one checks this lens, then the decision tree is wider. With compromise A, you get Samyang14mm f2.8, with compromise B the 17-40mm L is good, with compromise C sigma 12-24m.... the list goes on. Depends on what you can compromise and what you cannot.

the Samyang has better IQ than the Canon. But, the Samyang has more distortion and vignetting which can be corrected. Coma and CA can NOT be corrected. I like this Samyang as well on my 6d, razor sharp pics even at full f/2.8, and due to the short focal length, manual focus and control is not an issue for me.

You have a Sigma 10-20 ? I have had that lens and it should work on a fullframe like a 6d (not tried myself) and from 15mm on without vignetting. Lower FL (10-15mm) will obviously a circular image with a blurred edge.